[GHC] #8547: Data.Vector.foldl' hangs GHC indefinitely with -O2

[GHC]

#8547: Data.Vector.foldl' hangs GHC indefinitely with -O2
-----------------------------------+---------------------------------------
       Reporter:  blitzcode        |             Owner:
           Type:  bug              |            Status:  new
       Priority:  high             |         Milestone:
      Component:  Compiler         |           Version:  7.6.3
       Keywords:  hang vector      |  Operating System:  Unknown/Multiple
  fold'                            |   Type of failure:  Compile-time crash
   Architecture:                   |         Test Case:
  Unknown/Multiple                 |          Blocking:
     Difficulty:  Unknown          |
     Blocked By:                   |
Related Tickets:                   |
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 Running 'ghc -O2 -fforce-recomp -c Bug.hs' on the following

 {{{#!haskell
 module Bug where

 import qualified Data.Vector as V

 data Tree = Empty | Node !(V.Vector Tree)

 f :: k -> Tree -> Tree
 f k' (Node ch) = head $ V.foldl' (\u t' -> case t' of Empty -> u; _ -> t'
 : u) [] ch
 }}}

 will hang forever (or at least >5m). Builds fine with -O1 / V.foldl.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8547>
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